Used Panny lens on Ebay -- need advice based upon experience

It is best to move on. As some other posters have suggested, offer a return and get the lens back. Be certain to inspect it carefully and make certain the buyer did not do anything to it. At the end of the day, the buyer may be a jerk but it just isn't worth the hassle. If the buyer leaves negative feedback anyway, be certain to follow up with a comment (but don't be a jerk, yourself.)

Be certain that you do all your communication through eBay. That way, you will have a record of all the dialogue back and forth.

There are some people who are just a pain-in-the-a-- or try to rip off sellers, and even the best sellers get caught up by this once in a while. Fortunately, they are few in number.